1. Field of the Invention
The present invention relates, in general, to an antenna used in a radio communication. In particular, the invention relates to a composite antenna which can operable under a plurality of different frequency bands.
2. Description of the Related Art
Japanese laid-open patent application P2003-152445 discloses a conventional composite antenna which can operate under a plurality of different frequency bands. In this prior art, a circular polarized loop antenna structure for for 1.5 GHz band is formed on a dielectric substrate and a square patch antenna structure for 5.8 GHz band is also formed on the same substrate such that the patch antenna locates on the axis of the circular polarized loop antenna structure.
In recent years, an RFID (Radio Frequency Identification) system has been well known as one of the automatic identification technologies that utilize radio waves. The RFID system includes an interrogator (Reader/Writer) and a transponder (RFID tag) and a radio communication is carried out therebetween. When carrying out the radio communication, several transmission systems are used. One may be an electromagnetic coupling transmission that uses a mutual induction of coils caused by an alternating electromagnetic field. Another may be an electromagnetic induction transmission that uses a frequency below 135 kHz band or 13.56 MHz band. Still another may be a radio-wave transmission that uses a UHF band between 860 MHz and 960 MHz or 2.45 GHz band.
In particular, the electromagnetic induction transmission that utilizes 13.56 MHz band is used in a non-contact IC card system that is one of the applications of RFID system, and is widely adopted in many countries. The radio-wave transmission which utilizes a UHF band between 860 MHz and 960 MHz is approved to be used in European countries and the U.S.A, on the one hand, but is not approved in the RFID system in Japan, on the other hand.
Recently, a practical action has started in Japan to adopt a frequency band between 950 MHz and 956 MHz in RFID system and therefore development of a composite antenna that can be operable under not only 13.56 MHz band but also a frequency band between 950 MHz and 956 MHz is desired. That is, an RFID system that can be adapted to two different frequency bands has not been provided although such frequency bands are usable.